There is an increasing demand for longer intervals between changes of crankcase oils, and for a reduction in the volume of used oil to be disposed of. For these and other reasons, there is a need to improve the efficiency and useful life of oil-based lubricants, particularly those used as crankcase lubricants in internal combustion engines in automobiles and trucks.
One of the factors which substantially shortens the useful life of lubricating compositions is oxidation of the oil component. Oxidation results in the formation of acids, which tend to corrode engine parts, and in an undesirable increase in viscosity, which renders the composition less useful as a lubricant.
While high quality oils are themselves relatively resistant to oxidation, contaminants, for example iron, which are inevitably present in internal combustion engines, and common lubricant additives, for example magnesium- and/or calcium-containing detergents and alkenyl succinic acid/polyamine or polyester dispersants, have the effect of greatly accelerating the oxidation process, to the extent that oxidation is one of the major contributors to reduced lubricant life. Further, lower quality basestocks have a greater tendency to oxidize than do basestocks of higher quality.
Examples of oxidation inhibitors which have been proposed for used in crankcase lubricants are zinc dihydrocarbyl dithiophosphates which, although primarily used as antiwear agents, also act as antioxidants; aromatic amines, for example, alkylated diphenylamines and phenyl-oc-naphthylamines; hindered phenols; alkaline earth metal salts of sulphurized alkyl phenols in which the alkyl groups preferably contain 5 to 12 carbon atoms, for example, calcium nonylphenyl sulphide and barium octylphenyl sulphide; phosphosulphurized or sulphurized hydrocarbons; and oil-soluble copper compounds.
Some of the above-mentioned oxidation inhibitors have been found to be very effective in use. European Patent Specification No. 24 146 B claims lubricating compositions comprising a major amount of lubricating oil, from 1 to 10 wt. % of certain ashless dispersant compounds or from 0.3 to 10 wt. % of certain nitrogen- or ester-containing polymeric viscosity index improver dispersants, or mixtures of dispersant(s) and viscosity index improver dispersant(s), 0.01 to 5 wt. % of zinc dihydrocarbyl dithiophosphate (ZDDP) and 5 to 500 parts per million by weight of added copper in the form of an oil-soluble copper compound. For particularly severe conditions, where it may be desirable to use a supplementary antioxidant, the amount of the supplementary antioxidant required is small (far less than the amount required in the absence of the copper compound). Supplementary antioxidants mentioned in Specification No. 24 146 B include diphenylamine, alkylated diphenylamines, and phenyl-1-naphthylamine and its alkylated derivatives.
European Specifications Nos. 280 579A and 280 580A disclose the use of an oil-soluble copper compound, an oil-soluble sulphur-containing compound, and a bearing corrosion inhibitor in crankcase lubricating oils which are suitable for meeting modern requirements and which contain low or zero proportions of phosphorus. The oil-soluble sulphur-containing compounds disclosed include dithiocarbamates, polysulphides, and thiadiazoles. Borate esters and thiadiazole polysulphides are mentioned as bearing corrosion inhibitors.
U.S. Pat. Nos. 2,343,756 and 235,661 disclose the addition of oil-soluble copper compounds and oil-soluble sulphur-containing compounds to lubricating oils. The oil-soluble sulphur-containing compounds disclosed include polysulphides and thiocarbamates.
Despite the effectiveness of the copper-containing compositions disclosed in Specifications Nos. 24146, 280579 and 280580 discussed above, alternative highly effective antioxidants for lubricating compositions, particularly lubricating compositions suitable for use as crankcase lubricants containing, if desired, low or zero proportions of phosphorus, would represent an important contribution to the art. Phosphorus is known to have a deleterious effect on catalysts commonly used in catalytic converters used for emission control in automobiles.
It has now been found that copper and diamine sulphides, especially diamine polysulphides, provide surprisingly good oxidation control as the sole antioxidant system, or in combination with other antioxidants.